November 1970 Popular Electronics
[Table of Contents]People old and young enjoy waxing nostalgic about
and learning some of the history of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights (if any) are hereby acknowledged.
"One of the least known
but most fascinating of semiconductor devices is the light-emitting diode (LED). Until quite recently, these
devices were too expensive for widespread use." That statement is hard to imagine in 2011. It was written in late
1970, just 40 years ago. Now, LEDs seem to be in every consumer device, whether it be a simple power ON indicator
light or an array of alpha-numeric displays. TV and stereo remotes use infrared LEDs, optoisolators that use LEDs
are integral components of modems, motor controllers, and motion sensors. Commercial truck tail lights are made of
LED clusters, as are the grotesquely expensive LED bulbs that are verrrry slowly replacing incandescent and the
toxic (mercury) CFL household bulbs. What was once rare is now a commodity - it happens all the time.
all articles from
Light Emitting DiodesNew Semiconductors for Readout and Communication
story by Forrest M. Mims, III
of the least known but most fascinating of semiconductor devices is the light-emitting diode (LED). Until quite
recently, these devices were too expensive for widespread use; however, technological advances in their
fabrication now make possible moderately low prices so that they are attractive to the electronics experimenter.
The first recorded instance of light being generated by a "diode" was in 1907 when H. J. Round touched a pair of
battery wires to a crystal of silicon carbide. Much to his surprise, flashes of yellow light were emitted at the
contact region of one of the battery wires - he had accidentally discovered the LED. Unfortunately, his discovery
was forgotten and not until the early 1950's did scientists once again study semiconductor light emission. At that
time several patents were applied for covering LED's made from silicon or germanium - common semiconductor
materials. One of these patents not only described the principle of the LED but listed several fascinating uses
for the device. Among them were light-beam communication systems, light "radar", and light beam alignment devices.
Fig. 1. Most infrared LED's emit light at a wavelength of about 0.9 microns of the electromagnetic spectrum.
Visible LED's may emit from 0.53 microns to the limit of visible region at about 0.75 microns.
Unlike the 1907 silicon carbide cat whisker diode, the 1950's LED's emitted infrared light (see Fig. 1). The
invisible beam was desirable but researchers began a concentrated effort to fabricate LED's in the visible range.
The first company to market any kind of LED was Texas Instruments, Inc. Their diodes included infrared emitters
made from gallium arsenide (GaAs) and visible emitters made of gallium arsenide phosphide (GaAsP). These diodes
were expensive; but their appearance on the commercial market in 1962 whetted the appetites of design engineers -
and of course was of great interest to other semiconductor manufacturers. While scientists at IBM and Bell
Telephone performed basic research on the devices (especially visible emitters), General Electric, Monsanto,
Electro-Nuclear Laboratories and others began competing with TI.
Since the LED has an almost unlimited lifetime and because of its low operating current, many claims were
made for its potential in flat screen television, indicator lamps, night lights, and even as a source of room
lighting. The extremely fast modulation capability of the LED made possible several demonstrations of voice
communications permitting two parties to converse over a beam of invisible infrared light for clear weather
distances of several miles.
How Does It Do It?
The LED is different from conventional incandescent lamps because the
latter give off light as a byproduct of heat. That is, the filament must first be heated to incandescence before
light is produced. This, of course, is the reason for placing incandescent filaments in evacuated or gas-filled
glass bulbs. If the filament were exposed to oxygen, it would quickly be consumed. Not so with LED's; they operate
with or without the presence of air. In fact, the main reason LED's are placed in containers is to protect the
rather delicate contact wires. Often, in fact, the LED is simply protected by a layer of clear epoxy which serves
both as a protective cover and a lens. Since the LED produces far more light than heat, it is a more efficient
source of light than the incandescent lamp.
Production of light by an LED comes under the broad heading of
"electro-luminescence." Luminescence describes light produced by means other than incandescence. Since luminescing
bodies are generally at the temperature of their environment, they are sometimes referred to as sources of "cold
light." Fireflies, many species of fish, rotting wood, marsh gas, and many other objects are sources of
Light from an LED is the result of stimulation by a small electric current. As is the
case with all luminescent bodies, the light is a result of sub-atomic events. The cause of these events may be
chemical, as with the firefly, or electrical as with the LED.
. In the LED, emission of light is called "PN junction luminescence." As
shown in Fig. 2, light is emitted at the junction of an LED when electrons which have been stimulated to higher
than normal energy levels move across the junction and fall back into their normal places. The energy loss
resulting when an electron reoccupies its normal slot in an atom is accompanied by the emission of a photon of
light at a wavelength related to the difference in energy between the two bands. Since junction luminescence
involves the combination of electrons with holes, phvsicists often refer to the effect as "recombination
Fig. 2. Light is emitted when an electron crosses the np junction interface and falls from the excited to the
Fig. 3. The critical interface problem. Much more light is emitted by hemispheric diode but some is lost in the
thick n-region. Some planar emitters are provided with a reflector to give greater light output.
The sub-atomic physics of semiconductor light emission can be highly efficient. But the mechanisms of bringing the
light to the surface and the application of electrical current to the diode can be terribly inefficient. Sources
of inefficiency are:
1. Internal absorption of light within the p or n region of the semiconductor
following its emission at the junction.
2. Reabsorption of light which is reflected back into the diode due to the "critical interface" problem.
3. Resistance at the electrical contacts and in the semiconductor.
Many of these inefficiencies have been
partially resolved as a result of years of research. Consider, for example, the critical interface problem. Since
most semiconductors have a high index of refraction, they tend to reflect light at air-semiconductor interfaces.
The effect is similar to that of an observer looking through a glass window and seeing his own reflection as well
as objects on the other side of the window. The effect results in the loss of light that is generated at the
An ingenious solution to the problem was suggested by early research and was first used commercially by
Texas Instruments. It consists of forming the light emitting region of the diode into a dome. Since the light is
only bent and not reflected back into the diode, all light reaching the surface is emitted (see Fig. 3).
Unfortunately, the dome approach does have its drawbacks - the longer path that the light must travel before being
emitted causes more internal absorption than in planar emitters. Also, grinding domed diodes is an expensive
In an effort to gain the benefits of both planar and domed diodes, many manufacturers now coat
planar diodes with a transparent dome of epoxy which essentially serves the same purpose of the semiconductor
dome. PHYSICS OF LIGHT EMITTING DIODES
Modern atomic physics tells us that
electrons within the confines of a particular atom are allowed to occupy only discrete energy levels or bands.
Energy levels between the outer two bands, the valence and conduction bands, are separated by a region called the
forbidden gap. Under special conditions of doping, an electron may occupy one or more levels within the forbidden
gap for relatively brief periods of time. The forbidden gap plays an important role in semiconductor light
emission, since transitions from the conduction to the valence hand provide the mechanism for photon generation.
An efficient way to cause transitions between the valence and conduction bands is to pump or inject electrons into
the n region of a semiconductor diode. If a sufficient number of electrons is injected, the potential barrier at
the junction of the n and p regions will be overcome and current will flow through the diode. Having crossed the
junction barrier, the injected electrons seek their equilibrium point and drop from their excited position in the
conduction band to the valence band. In essence, the electrons drop into holes or regions in a band where there is
an electron deficiency.
The act of electrons combining with holes is called recombination and results in the magic of PN junction
electroluminescence or light emission. An electron falling from a high to a low level releases the energy which
propelled it over the junction as heat, light, or some of each. Heat emission is accompanied by vibrations in the
crystal lattice and if left uncontrolled, results in thermal destruction of the diode. Heat emission predominates
in indirect band gap semiconductors such as silicon and germanium. The forbidden gap in such materials permits an
electron falling from the conduction to the valence band to loiter briefly at one or more levels. Transitions
between the various levels in the forbidden gap may result in either light or heat emission.
interesting to note that an ordinary silicon or germanium diode emits a minute quantity of infrared light when
forward biased. However, light emission in such diodes is far less efficient than heat production and development
of practical semiconductor light emitters followed research with direct band-gap materials.
arsenide GaAs is normally a direct band-gap material. Since electrons injected into a GaAs diode recombine with
holes without pausing at intermediate levels within the forbidden gap, light emission is very efficient and
production of heat is generally limited to contact resistance and bulk absorption of some of the emitted light
within the semiconductor.
GaAs diodes are so efficient as light emitters that scientists chose them as candidates for early work
concerning the feasibility of fabricating semiconductor lasers. In the fall of 1962, researchers at G.E., IBM, and
MIT announced almost simultaneously lasers made from specially prepared GaAs diodes. In a future issue of POPULAR
ELECTRONICS, the operating principles and characteristics of the diode laser will be described.
How Are They Made?
As mentioned earlier, the LED usually consists of a PN junction of one type of
semiconductor, the most common being gallium arsenide. A typical diode may consist of a small wafer of positively
charged (p) material in intimate electrical contact with a metal header similar to a transistor case. Before it is
mounted on the header, the wafer is given a thin top layer of negatively charged (n) GaAs by either diffusion or
epitaxial growth. The P portion of the finished wafer is connected to one of the wire leads of the header,
resulting in a true PN junction diode.
To collect the light output from the wafer in an efficient manner,
a metal can with a lens or flat window at one end is welded to the header. The can also serves as protection (see
How Are They Used?
Fig. 4. In practical applications, the LED is mounted within a metal enclosure, similar to a transistor, for
mechanical protection. A transparent window permits the light to exit from one end of the enclosure.
Now that we know something about the history, physics, and mechanical
construction of the LED, what are its applications other than those found in the laboratory? Especially, how can
the experimenter use these tiny sources of "cold light."
An important military use of the LED is as a covert source of illumination for night vision devices. The
usefulness of the Army's starlight scope is greatly enhanced by invisible infrared illumination supplied by one or
more LED's. Another military use is in covert communications. The Navy, for example, has several types of
infrared, line-of-sight LED voice communicators for ship-to-ship and ship-toshore.
applications of the LED have been revealed at an unprecedented rate. Several companies are mass producing tiny
alpha-numeric displays consisting of arrays of LED's. These displays operate at a much lower voltage than
conventional mechanical, incandescent, cathode ray, or gas-discharge displays.
One manufacturer has
demonstrated a complete VOM in a probe! Using several of the new numeric displays, the futuristic unit is similar
in size to a standard scope probe. Volts and ohms are read directly from the small light display. The Hamilton
Watch Company will market a digital watch using LED numeric displays, and Bell Telephone is hard at work on new
types of blue, green, yellow, and red LED's for use in home telephones. Infrared LED's are being used in several
types of new intrusion alarms. One company even markets a complete line of LED voice communicators and burglar
One of the most unusual applications of the infrared LED is as a light source for small mobility aids for the
blind. Such devices may eventually be marketed at a price comparable to most hearing aids.
result of all these new uses for LED's is a large drop in price. LED's are available for under $2.50 each in small
quantities. This price is competitive with that of miniature, long-life indicator lamps. And of course, the LED
offers sturdier packaging, a million-hour lifetime, and far less current consumption.
Only a year ago, the
cheapest infrared LED retailed for $18.00. At least five manufacturers now offer GaAs LED's for under $7.50. These
devices are far more efficient than visible LED's and are therefore usable in experiments in secret
To sum up then, the LED is superior to the conventional filament lamp in the following
1. Response time is extremely fast most LED's operate in a fraction of a second rise and fall time
. Some LED' can reach a speed of 100 MHz.
2. Because it is a solid-state device, an LED has no warm-up
time, is completely free of microphonics, is just about impervious to mechanical vibration and other environmental
conditions, and is usually of very small size and weight.
3. The LED light output is nearly
monochromatic-far from being a laser but close enough to have most of its light in a relatively narrow
bandwidth. This makes it possible to use optical filters to reduce ambient noise.
4. The LED is a low-impedance device with forward characteristics similar to those of a conventional silicon
diode and it can be driven from ordinary low-voltage supplies with conventional transistor circuitry.
Elsewhere in this issue you will find a unique construction article for a low-cost LED communicator. Other
projects using LED's will appear in future issues.
For more information about commercial light emitting diodes, write to
one of the following manufacturers:
- Electro·Nuclear Laboratories 115 Independence Drive Menlo Park, CA 94025
- Miniature Lamp Department General Electric Company Nela Park Cleveland, OH 44112
- Monsanto Electronic Special Products 10131 Bubb Road Cupertino, CA 95014
- Radio Corporation of America Electronic Components and Devices Harrison, NJ 07029
- Texas Instruments Semiconductor Components Division Box 5012 Dallas, TX 75222
- Fairchild Semiconductor 313 Fairchild Drive Mountain View, CA 94040
- Hewlett-Packard 1501 Page Mill Road Palo Alto, CA 94304
- Motorola Semiconductor Products Box 20912 Phoenix, AZ 85036
- Sharp Electronics Corporation 178 Commerce Road Carlstadt, NJ 07072
- An excellent booklet on LED's is sold by the General Electric Company for $2.00. Called the "Solid State
Lamp Manual," it describes in detail theory, characteristics, and applications.